The present invention relates to a curing agent for epoxy resin, an epoxy resin composition and a method for preparing a siloxane-modified phenol resin.
Epoxy resins have been used usually in combination with curing agents. Particularly, phenol novolac resins have been suitably used as curing agents for epoxy resin in the field of electric and electronic materials because of their excellent heat resistance, chemical resistance, electrical characteristic, etc. However, the recent development in the electric and electronic material field has been requiring high-performance epoxy resin compositions. Therefore, the epoxy resin compositions containing the phenol novolac resins as curing agents do not have sufficient heat resistance.
In order to improve the heat resistance of the epoxy resin compositions, glass fibers, glass particles, mica and like fillers are added to epoxy resins in addition to phenol novolac resins used as curing agents. However, these methods using fillers can not impart sufficient heat resistance to the resin compositions. By these methods, the transparency of the cured epoxy resin composition is deteriorated and the interfacial adhesion between the fillers and resins is lowered. Thus, the cured epoxy resin compositions are given insufficient mechanical properties such as elongation rate.
Japanese Unexamined Patent Publication No. 1997-216938 proposes a method for improving the heat resistance of cured epoxy resin composition. In this method, the complex of a phenol novolac resin and silica prepared by hydrolysis and condensation of alkoxysilane in the presence of a phenol novolac resin is used as a curing agent for epoxy resin. The heat resistance of the cured epoxy resin composition comprising such complex as a curing agent is improved to some extent. However, water contained in the curing agent or water and alcohols such as methanol produced during curing cause voids (air bubbles) inside the cured product. Further, increasing the amount of alkoxysilane to further improve the heat resistance of the cured product results in impaired transparency and whitening of the product due to silica aggregation. In addition, solation of a large amount of the alkoxysilane necessitates a large amount of water, which leads to the bends and cracks in the cured product.
An object of the present invention is to provide a novel phenol resin-based curing agent for epoxy resin which is free from the aforementioned problems of the prior art.
Another object of the present invention is to provide a novel epoxy resin composition which is capable of providing cured products having high heat resistance and no voids (air bubbles) or cracks using a specific phenol resin-based curing agent for epoxy resin.
Another object of the present invention is to provide a novel method for preparing a siloxane-modified phenol resin which is useful as a phenol resin-based curing agent for epoxy resin.
Other objects and features of the present invention will be apparent from the following description.
The present invention provides a curing agent for epoxy resin, the curing agent containing a siloxane-modified phenol resin (3) obtained by dealcoholization condensation reaction between a phenol resin (1) and hydrolyzable alkoxysilane (2).
The present invention also provides an epoxy resin composition comprising an epoxy resin and the above curing agent for epoxy resin.
Further, the present invention provides a method for preparing a siloxane-modified phenol resin (3) characterized by subjecting a phenol resin (1) and hydrolyzable alkoxysilane (2) to dealcoholization condensation reaction.
The inventors of the present invention conducted extensive research to solve the above-mentioned problems of the prior art. Consequently, the inventors found the following: by using the epoxy resin composition comprising a siloxane-modified phenol resin obtained by dealcoholization condensation reaction of a phenol resin and hydrolyzable alkoxysilane as a curing agent for epoxy resin, a cured product having high heat resistance and no voids (air bubbles) and cracks can be obtained. The present invention was accomplished based on this novel finding.
In the present invention, the phenol resins (1) forming the siloxane-modified phenol resin (3) may be any of novolac phenol resins and resol phenol resins. The former can be prepared by reacting a phenol compound and an aldehyde compound in the presence of an acid catalyst, and the latter by reacting a phenol compound and an aldehyde compound in the presence of an alkaline catalyst. The resol phenol resins usually contain condensation water, which may cause hydrolysis of the hydrolyzable alkoxysilane (2). Therefore, the novolac phenol resins are preferably used in the present invention. The phenol resins (1) preferably have an average phenolic unit number of about 3 to about 8.
Examples of useful phenol compounds mentioned in the above include phenol, o-cresol, m-cresol, p-cresol, 2,3-xylenol, 2,4-xylenol, 2,5-xylenol, 2,6-xylenol, 3,4-xylenol, 3,5-xylenol, p-ethylphenol, p-isopropylphenol, p-tertiary butyl phenol, p-chlorophenol and p-bromophenol. Examples of useful formaldehyde compounds include formalin and formaldehyde-generating compounds such as paraformaldehyde, trioxane and tetraoxane. As the acid catalyst or alkaline catalyst, any of those conventionally known is useful.
An example of the hydrolyzable alkoxysilane (2) forming the siloxane-modified phenol resin (3) in the present invention is a compound represented by the following formula or the partial condensate thereof:
R1nSi(OR2)4xe2x88x92n
(wherein n is an integer of 0 to 2; R1 represents a lower alkyl group which may have a functional group directly bonded to a carbon atom, an aryl group or an unsaturated aliphatic hydrocarbon group; when n is 2, the two R1""s may be the same or different; R2 represents a hydrogen atom or a lower alkyl group and may be the same or different, with the proviso that at least one R2 group is a lower alkyl group.). Examples of the above functional group include a vinyl group, a mercapto group, an epoxy group and a glycidoxy group. The term lower alkyl group indicates a straight-chain or branched-chain alkyl group having 6 carbon atoms or less.
The hydrolyzable alkoxysilane (2) may be suitably selected from the compounds represented by the above formula or their partial condensates, and may be used singly or at least two of them in mixture. However, the hydrolyzable alkoxysilane (2) is preferably a partial condensate whose average number of Si per molecule is about 2 to about 100. The hydrolyzable alkoxysilane whose average number of Si is less than 2 suffers an increase in the amount of unreacted alkoxysilane discharged together with the alcohol from the reaction system during the dealcoholization condensation reaction with the phenol resin (1). When the average number of Si is 100 or greater, the reactivity of the alkoxysilane with the phenol resin (1) is decreased and thus the desired substance is hard to obtain. Because of the availability of commercial products, the average number of Si per molecule may be about 3 to about 20.
Examples of the hydrolyzable alkoxysilane (2) include tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, tetraisopropoxysilane, tetrabutoxysilane and like tetraalkoxysilanes; methyltrimethoxysilane, methyltriethoxysilane, methyltripropoxysilane, methyltributoxysilane, ethyltrimethoxysilane, ethyltriethoxysilane, n-propyltrimethoxysilane, n-propyltriethoxysilane, isopropyltrimethoxysilane, isopropyltriethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, 3-glycidoxypropyltrimethoxysilane, 3-glycidoxypropyltriethoxysilane, 3-mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane, phenyltrimethoxysilane, phenyltriethoxysilane, 3,4-epoxycyclohexylethyltrimetboxysilane, and like trialkoxysilanes; dimethyldimethoxysilane, dimethyldiethoxysilane, diethyldimethoxysilane, diethyldiethoxysilane and like dialkoxysilanes; and the partial condensates of these compounds.
Among these compounds, preferably used is a partial condensate of at least one member selected from the group consisting of tetraalkoxysilanes and trialkoxysilanes because of their higher rates of the dealcoholization condensation reaction and curing reaction. More preferable is a partial condensate of at least one member selected from the group consisting of tetramethoxysilane and methyltrimethoxysilane.
The siloxane-modified phenol resin (3) in the present invention is prepared by the dealcoholization condensation reaction between the phenol resin (1) and hydrolyzable alkoxysilane (2). This reaction forms the siloxane-modified phenol resin (3) by modifying part of the phenolic hydroxyl group of the phenol resin (1) with the hydrolyzable alkoxysilane (2).
The ratio of the phenol resin (1) to the hydrolyzable alkoxysilane (2) used in this reaction may be such that the resulting siloxane-modified phenol resin (3) contains the phenolic hydroxyl group which acts as a curing agent for epoxy resin. Preferably, the equivalent ratio of phenolic hydroxyl groups of the phenol resin (1) to alkoxy groups of the hydrolyzable alkoxysilane (2) is 0.2 to 10. However, when this equivalent ratio is about 1 (approximately equal in stoichiometry), the dealcoholization reaction is accelerated and therefore thickening and gelation of the solution may occur. Thus, in this case, the progress of the dealcoholization reaction needs to be controlled.
When the above ratio is less than 1, the proportion of the hydrolyzable alkoxysilane (2) in the siloxane-modified phenol resin (3) increases. Since this hydrolyzable alkoxysilane is a curing agent for the epoxy resin, the silica content in the cured epoxy resin composition increases, effectively improving the heat resistance and hardness of the cured product. For example, when using a high molecular epoxy resin having an epoxy equivalent of 400 or greater, the crosslink density of the resulting cured epoxy product is usually lowered. In this case, the suitable ratio is less than 1. However, when the ratio is extremely low, the amount of the phenolic hydroxyl group in the siloxane-modified phenol resin (3) is reduced. This may lead to a decrease in the curing reactivity of the siloxane-modified phenol resin with the epoxy resin, insufficient crosslink density in a cured product and an increase in the proportion of the unreacted hydrolyzable alkoxysilane (2), whereby the whitening of the cured product may be caused. For this reason, the equivalent ratio is preferably 0.2 or higher, more preferably 0.3 or higher.
When the equivalent ratio of phenolic hydroxyl groups to alkoxy groups is greater than 1, the amount of alkoxy groups of the hydrolyzable alkoxysilane (2) remaining in the siloxane-modified phenol resin (3) is lowered. When the epoxy resin is cured with this siloxane-modified phenol resin, alcohols such as methanol and water are hardly produced as a by-product by the condensation reaction of the alkoxysilyl group. Therefore, the formation of bends, voids (air bubbles) and cracks in the cured product can be effectively prevented. Such siloxane-modified phenol resin (3) is useful as a curing agent, for example, for novolac phenol epoxy resin and epoxy resin having an epoxy equivalent less than 400 (particularly an epoxy equivalent of 200 or less) whose cured products are prone to bends, voids (air bubbles) and cracks. When the equivalent ratio is too high, the silica content in the curing agent is decreased and the heat resistance of the cured epoxy resin composition can not be sufficiently improved. Hence, the equivalent ratio is preferably 10 or lower, more preferably 8 or lower.
The siloxane-modified phenol resin (3) is prepared, for example, by mixing the phenol resin (1) and the hydrolyzable alkoxysilane (2), heating the mixture to remove alcohol formed by dealcoholization condensation reaction. The reaction temperature is about 70xc2x0 C. to about 150xc2x0 C., preferably about 80xc2x0 C. to about 110xc2x0 C. The total reaction time is about 2 hours to about 15 hours. This reaction is preferably performed under substantially anhydrous conditions to prevent the condensation reaction of the hydrolyzable alkoxysilane (2) itself.
In the dealcoholization reaction, conventionally known catalysts may be used to accelerate the reaction. Examples of the catalyst include acetic acid, p-toluenesulfonic acid, benzoic acid, propionic acid and like organic acids; lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, barium, strontium, zinc, aluminum, titanium, cobalt, germanium, tin, lead, antimony, arsenic, cerium, boron, cadmium, manganese and like metals; oxides, organic acid salts, halides, alkoxides and the like of these metals. Among these, organic acids, organotin, tin organoate are particularly preferable. More specifically, acetic acid, dibutyltin dilaurate, tin octoate, etc., are effectively used.
The above reaction can be performed in a solvent or without a solvent. The solvent is not particularly limited insofar as it can dissolve the phenol resin (1) and hydrolyzable alkoxysilane (2). Examples of such solvent include dimethylformamide, dimethylacetamide, methyl ethyl ketone and cyclohexanone. If the rapid progress of dealcoholization reaction is desired, the reaction is preferably performed without the solvent. However, it is favorable to use the solvent when the viscosity of the reaction system is excessively increased for the following reasons: the equivalent ratio of phenolic hydroxyl groups of the phenol resin (1) and alkoxy groups of the hydrolyzable alkoxysilane (2) is about 1; the hydrolyzable alkoxysilane (2) has an average Si number per molecule of 8 or greater.
In the above reaction, in order to obtain the siloxane-modified phenol resin having the desired phenolic hydroxyl equivalent and viscosity, the dealcoholization reaction between the phenol resin (1) and hydrolyzable alkoxysilane (2) may be stopped in the course of the reaction. The method for stopping this reaction is not critical. For example, effective methods are cooling, deactivating the catalyst or adding alcohol to the reaction system upon obtaining the desired amount of alcohol effluent.
The thus-obtained siloxane-modified phenol resin (3) of the present invention contains, as a main component, the phenol resin having at least one of the phenolic hydroxyl groups is modified with silane. The resin (3) may contain unreacted phenol resin (1) and hydrolyzable alkoxysilane (2). Like the normal phenol resin, the unreacted phenol resin (1) acts as a curing agent for epoxy resin. The remaining hydrolyzable alkoxysilane (2) can be converted to silica by hydrolysis and condensation. To promote the hydrolysis and condensation, a catalyst may be added to the siloxane-modified phenol resin (3). The catalyst is selected from a small amount of water; a catalytic amount of formic acid, acetic acid, propionic acid, para-toluenesulfonic acid, methanesulfonic acid and like organic acid catalyst; boric acid, phosphoric acid and like inorganic catalyst; alkaline catalyst; organotin, tin organoate catalyst.
In the present invention, the siloxane-modified phenol resin (3) is used as a curing agent for epoxy resin. The alkoxysilyl group in the siloxane-modified phenol resin (3) of the present invention undergoes hydrolysis and condensation upon contacting water, forming a siloxane bond. Thus, the siloxane-modified phenol resin (3) is subject to changes in molecular weight and viscosity because of external water such as moisture in air. To avoid this effectively, when the siloxane-modified phenol resin (3) is left in an open system for a long time or stored in a humid environment, an alcohol solvent such as methanol may be added to the siloxane-modified phenol resin (3) after the reaction between the phenol resin (1) and the hydrolyzable alkoxysilane (2) is completed.
The epoxy resin composition of the invention is usually prepared by using an epoxy resin and the siloxane-modified phenol resin (3) as a curing agent for epoxy resin. The equivalent ratio of hydroxyl groups of the curing agent to epoxy groups of the epoxy resin may be in the range of about 0.5 to about 1.5.
The epoxy resin may be any of those known conventionally. Examples of the epoxy resin include orthocresol novolac epoxy resin, phenol novolac epoxy resin and like novolac epoxy resin; bisphenol A, bisphenol F and like diglycidyl ethers; glycidyl ester epoxy resin obtainable by reacting phthalic acid, dimer acid and like polybasic acids with epichlorohydrin; glycidyl amine epoxy resin obtainable by reacting diaminodiphenylmethane, isocyanuric acid or like polyamines with epichlorohydrin; and linear aliphatic epoxy resin and alicyclic epoxy resin obtainable by oxidizing olefin bond with peracetic acid and like peracids. These epoxy resins may be used singly or in combinations of two or more types.
The epoxy resin composition may contain an accelerator for curing between epoxy resin and a curing agent. Examples of the accelerator include 1,8-diazabicyclo[5.4.0]undecene-7, triethylenediamine, benzyl-dimethylamine, triethanolamine, dimethylaminoethanol, tris(dimethylaminomethyl)phenol and like tertiary amines; 2-methylimidazole, 2-phenylimidazole, 2-phenyl-4-methylimidazole, 2-heptadecylimidazole and like imidazoles; tributylphosphine, methyl diphenylphosphine, triphenyl phosphine, diphenyl phosphine, phenyl phosphine and like organic phosphines; and tetraphenylphosphonium.tetraphenyl borate, 2-ethyl-4-methylimidazole-tetraphenyl borate, N-methylmorpholine.tetraphenyl borate and like tetraphenyl borates. The accelerator is preferably used in an amount of 0.1 to 5 parts by weight relative to 100 parts by weight of the epoxy resin.
The concentration of the epoxy resin composition can be suitably controlled using a solvent. The solvent may be the same as that used for preparing the siloxane-modified phenol resin. The epoxy resin composition may also contain fillers, mold releasing agents, finishing agents, fire retardants, etc., if necessary.
The curing agent for epoxy resin of the present invention can provide an epoxy resin composition for producing cured products with high heat resistance and without voids (air bubbles).
The epoxy resin composition of the present invention is useful as a IC sealing material, an epoxy resin laminate plates, a coating for electric/electronic materials, a coating composition, inks and the like.